What this code means
The Navien manual for this product family lists E421 as: Cold water inlet thermistor open or short.
Safe checks
- Record the exact Navien code, whether the unit is locked out, and when the code appeared.
- Confirm thermostat, controller, or front-panel status without changing service settings.
- Check only visible intake, exhaust, filter, clearance, condensate, and water-supply conditions that can be inspected without opening panels.
- Take photos of the code, equipment nameplate, venting, surrounding equipment area, thermostat or controller, and any visible leak, ice, or obstruction.
First step
Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains.
Frequently asked questions
What does E421 mean on a Navien NCB-240/110H?
The Navien manual for this product family lists E421 as: Cold water inlet thermistor open or short.
What should I safely check before scheduling service?
Use only the visible, non-invasive checks listed on this page. Do not open equipment panels.
When should GreenFlow handle this code?
Call GreenFlow if the code repeats, the unit is locked out, hot water, heating, or cooling is not restored, or the concern involves gas, flame, combustion, venting, electrical components, refrigerant, pressure, water leaks, PCB/control boards, sensors, pumps, or warranty coverage.